Different types of membranes are used for gas separation and water purification. Porous and non-porous membranes are widely used for gas separation and water purification. A membrane is largely determined by membrane transport properties. The membrane transport properties include permeability of the membrane and selectivity for a specific molecule in a mixture.
The porous membranes are rigid, high voided with randomly distributed pores. The porous membranes which are used for separation are specifically designed to preferentially separate molecules in a mixture. For example, porous membranes with pore diameter smaller than the mean free path of a gas molecule are designed. The separation of gas using the porous membranes is based on pore size and pore distribution. The gas molecules can be separated using porous membrane only if sizes of gas molecules differ considerably in a gas mixture. Further, the porous membranes exhibit high level of flux but inherit low selectivity values. Thus, the porous membranes have high permeability but low selectivity properties.
The non-porous membranes are designed to adsorb specific molecules from a mixture on to the solid surface of the non-porous membranes. For example, a non-porous membrane prepared using mineral zeolite specifically adsorbs carbon dioxide (CO2) from a gas stream. Thus, the porous membranes have high selectivity but low permeability properties.
When permeability of a membrane increases, selectivity decreases and when the selectivity increases, the permeability decreases. Therefore, there is a need to develop an improved membrane for gas separation and water purification.